My 68 Dart has normal alt. readings during daylight driving but at
night or during rain it seems like there is too much drain on the alt.
Running with headlights or wipers really pulls the alt. down at idle.
I can hear the turn signals slow down when I come to a stop and see
the lights dim.

Do I need to rebuild the alt. (original equip.)? I have owned cars this old
and older but only had this type of drain on my 66 Bug.

Yes, you may have a problem with your alternator or its circuitry. Could be as simple as needing new alternator brushes. Some dimming at idle with lights on is normal, but not a huge "brownout" and slowdown of the wipers, fan, etc. This topic has been covered recently.

Also, test your battery. A battery with a dead/shorted cell will often still start the car, and the alternator will keep up with the increased demand above idle, but not at idle with lights and wipers going.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Could also be 40 year old wiring showing it's age. JMO, but I have found the alt output wire on every pre 80's Dodge (admittedly mostly Dodge Trucks) a little small for my taste, even if its only for a 60 amp alternator.

Fairly easy to check with a voltmeter. Check the voltage at the alt output stud (engine running). Then check battery voltage (engine still running). I personally prefer these numbers be within ~0.3 volts of each other. To achieve such a low voltage drop, I wire for 'overkill'. I use 6 gauge wire between my 95 amp alt and battery, and have virtually no voltage drop; a DVM shows about .05 volts difference on my truck.
10 gauge would probably be sufficient for a 40 or 60 amp alt output wire, but I would suggest at least 8 gauge.

$0.02

-S/6

Thanks all, I'll check the wire for voltage drop.

I have also heard that replacement alts. are not to spec. with original
equip. (Chinese or Japanese versions) I have heard run at the wrong
RPM due to wrong pully size.

Would I be better off getting the original alt. rebuilt? I had a 77 aspen /6
that had a blown diode in the alt. Being poor at the time I bought a diode
and soldered (it was not pretty) it in. Surprisingly it worked for 2 years
till the bearings went south.

I had this same problem with my '74 Dart. Turned out to be a shorted field in the alternator. If you buy a rebuilt alternator you will likely get original equipment rebuilt with Chinese parts, If you buy a new one the whole thing will likely be made in China.